1. Field of the Invention
The invention relates to methods and systems for the treatment of exhaust gas generated by xe2x80x9clean burnxe2x80x9d operation of an internal combustion engine which are characterized by reduced tailpipe emissions of a selected exhaust gas constituent.
2. Background Art
Generally, the operation of a vehicle""s internal combustion engine produces engine exhaust that includes a variety of constituent gases, including carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx). The rates at which the engine generates these constituent gases are dependent upon a variety of factors, such as engine operating speed and load, engine temperature, ignition (xe2x80x9csparkxe2x80x9d) timing, and EGR. Moreover, such engines often generate increased levels of one or more constituent gases, such as NOx, when the engine is operated in a lean-burn cycle, i.e., when engine operation includes engine operating conditions characterized by a ratio of intake air to injected fuel that is greater than the stoichiometric air-fuel ratio, for example, to achieve greater vehicle fuel economy.
In order to control these vehicle tailpipe emissions, the prior art teaches vehicle exhaust treatment systems that employ one or more three-way catalysts, also referred to as emission control devices, in an exhaust passage to store and release selected exhaust gas constituents, such as NOx, depending upon engine operating conditions. For example, U.S. Pat. No. 5,437,153 teaches an emission control device which stores exhaust gas NOx, when the exhaust gas is lean, and releases previously-stored NOx when the exhaust gas is either stoichiometric or xe2x80x9crichxe2x80x9d of stoichiometric, i.e., when the ratio of intake air to injected fuel is at or below the stoichiometric air-fuel ratio. Such systems often employ open-loop control of device storage and release times (also respectively known as device xe2x80x9cfillxe2x80x9d and xe2x80x9cpurgexe2x80x9d times) so as to maximize the benefits of increased fuel efficiency obtained through lean engine operation without concomitantly increasing tailpipe emissions as the device becomes xe2x80x9cfilled.xe2x80x9d
The timing of each purge event must be controlled so that the device does not otherwise exceed its capacity to store the selected exhaust gas constituent, because the selected constituent would then pass through the device and effect an increase in tailpipe emissions. Further, the timing of the purge event is preferably controlled to avoid the purging of only partially filled devices, due to the fuel penalty associated with the purge event""s enriched air-fuel mixture. Moreover, when plural emission control devices are deployed in series, excess feedgas HC and CO during the purge event are typically initially consumed in the upstream device to release stored oxygen, whereupon the excess feedgas HC and CO ultimately xe2x80x9cbreak throughxe2x80x9d the upstream device and enter the downstream device to thereby effect a both an initial release of oxygen previously stored in the downstream device and then a release of stored selected exhaust gas constituent.
The prior art has recognized that the storage capacity of a given emission control device is itself a function of many variables, including device temperature, device history, sulfation level, and the presence of any thermal damage to the device. Moreover, as the device approaches its maximum capacity, the prior art teaches that the incremental rate at which the device continues to store the selected constituent, also referred to as the instantaneous efficiency of the device, may begin to fall. Accordingly, U.S. Pat. No. 5,437,153 teaches use of a nominal NOx-retaining capacity for its disclosed device which is significantly less than the actual NOx-storage capacity of the device, to thereby provide the device with a perfect instantaneous NOx-absorbing efficiency, that is, so that the device is able to absorb all engine-generated NOx as long as the cumulative stored NOx remains below this nominal capacity. A purge event is scheduled to rejuvenate the device whenever accumulated estimates of engine-generated NOx reach the device""s nominal capacity. Unfortunately, however, the use of such a fixed nominal NOx capacity necessarily requires a larger device, because this prior art approach relies upon a partial, e.g., fifty-percent NOx fill in order to ensure retention of engine-generated NOx.
The amount of the selected constituent gas that is actually stored in a given emission control device during vehicle operation depends on the concentration of the selected constituent gas in the engine feedgas, the exhaust flow rate, the ambient humidity, the device temperature, and other variables including the xe2x80x9cpoisoningxe2x80x9d of the device with certain other constituents of the exhaust gas. For example, when an internal combustion engine is operated using a fuel containing sulfur, the prior art teaches that sulfur may be stored in the device and may correlatively cause a decrease in both the device""s absolute capacity to store the selected exhaust gas constituent, and the device""s instantaneous constituent-storing efficiency. When such device sulfation exceeds a critical level, the stored SOx must be xe2x80x9cburned offxe2x80x9d or released during a desulfation event, during which device temperatures are raised above perhaps about 650xc2x0 C. in the presence of excess HC and CO. By way of example only, U.S. Pat. No. 5,746,049 teaches a device desulfation method which includes raising the device temperature to at least 650xc2x0 C. by introducing a source of secondary air into the exhaust upstream of the device when operating the engine with an enriched air-fuel mixture and relying on the resulting exothermic reaction to raise the device temperature to the desired level to purge the device of SOx.
Thus, it will be appreciated that both the device capacity to store the selected exhaust gas constituent, and the actual quantity of the selected constituent stored in the device, are complex functions of many variables that prior art accumulation-model-based systems do not take into account. The inventors herein have recognized a need for a method and system for controlling an internal combustion engine whose exhaust gas is received by an emission control device which can more accurately determine the amount of the selected exhaust gas constituent, such as NOx, stored in an emission control device during lean engine operation and which, in response, can more closely regulate device fill and purge times to optimize tailpipe emissions.
Under the invention, a method is provided for controlling an engine operating over a range of operating conditions including those characterized by combustion of air-fuel mixtures that are both lean and rich of a stoichiometric air-fuel ratio, and wherein exhaust gas generated during engine operation is directed through an exhaust purification system including an upstream emission control device and a downstream sensor operative to generate an output signal representing a concentration of reductants, i.e., excess hydrocarbons, in the exhaust gas exiting the device. The method includes determining a first value representing a cumulative amount of a selected constituent of the engine feedgas, such as NOx, generated during an engine operating condition characterized by combustion of an air-fuel mixture lean of the stoichiometric air-fuel ratio (xe2x80x9ca lean operating conditionxe2x80x9d). The method also includes determining a second value representing an instantaneous capacity of the device to store the selected constituent, wherein the second value is determined as a function of a characteristic of the output signal generated by the reductant sensor during an engine operating condition characterized by combustion of an air-fuel mixture having an air-fuel ratio rich of the stoichiometric air-fuel ratio (xe2x80x9ca rich air-fuel ratioxe2x80x9d), and a predetermined reference value. The method further includes selecting an engine operating condition as a function of the first and second values.
More specifically, in a preferred embodiment in which the selected exhaust gas constituent is NOx, the first value is estimated using a lookup table containing mapped values for engine-generated NOx as a function of engine operating conditions, such as instantaneous engine speed and load, air-fuel ratio, spark and EGR. The lean operating condition is discontinued, and a rich operating condition suitable for purging the device of stored feedgas NOx is scheduled, when the first value representing accumulated feedgas NOx exceeds the second value representing the instantaneous device NOx-storage capacity. The second value is a previously stored value which is periodically adaptively updated based upon a comparison of the amplitude of the reductant sensor""s output signal with the predetermined reference value during a subsequent device purge event. In this manner, the storage of NOx by the device and, hence, the xe2x80x9cfill timexe2x80x9d during which the engine is operated in a lean operating condition, is optimized.
In accordance with another feature of the invention, the method preferably includes calculating a third value representing the amount of fuel, in excess of a stoichiometric amount, which is necessary to purge the device of both stored selected exhaust gas constituent and stored oxygen, based on the first value representing accumulated exhaust gas constituent present in the engine feedgas and a previously stored fourth value representing the amount of excess fuel necessary to purge only stored oxygen from the device. The method also preferably includes determining a fifth value representing a cumulative amount of fuel, in excess of the stoichiometric amount, which has been supplied to the engine during a given rich operating condition; and discontinuing the purge event when the fifth value representing the supplied excess fuel exceeds the third value representing the necessary excess fuel to purge the device of all stored selected constituent and stored oxygen. In this manner, the invention optimizes the amount of excess fuel used to purge the device and, indirectly, the device purge time.
In accordance with another feature of the invention, the method preferably includes selecting an engine operating condition suitable for desulfating the device when the second value representing the device""s instantaneous capacity to store the selected exhaust gas constituent falls below a minimum threshold value. The method further preferably includes indicating a deteriorated device if a predetermined number of device-desulfating engine operating conditions are performed without any significant increase in the second value.
In accordance with a further feature of the invention, the fourth value representing the oxygen-only excess fuel amount is periodically updated using an adaption value which is itself generated by comparing the amplitude of the reductant sensor""s output signal to a threshold value during a scheduled purge.
The above object and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.